Fibercement has been used in the United States building materials industry since the 1980's. This material is used in residential and commercial construction applications as an alternative to wood for siding, roofing, backer board, trim and fascia applications. Fibercement is fire and insect resistant, and is more durable. In fact, it was the fastest growing market segment in the exterior sheathing industry in the 1990's, and by 2005, this material is expected to gain up to 25 percent of the siding market.
Fibercement is, technically, a composite of portland cement, aggregate (usually sand), and cellulose fibers. Cellulose fibers are added to cement to increase its toughness and crack-arresting ability. Fibercement shingle and shake products are widely available from such sources as James Hardie, Inc. under the brand name Hardiplank® and CertainTeed Corporation under the brand name Weatherboards™. These products are produced by the Hatchek de-watering process, which results in a laminated flat sheet reinforced with a significant amount of cellulose fibers, usually about 30-35 percent by volume.
Fibercement materials possess useful properties, but they were at one point in their history believed to be unsuitable for exterior use since they were susceptible to damage due to the effect of freeze-thaw cycles. See Harper et al., U.S. Pat. No. 4,637,860. Freeze-thaw action can cause severe deterioration to fibercement building products. The primary cause of damage is due to the hydraulic pressures that develop as water freezes and expands in tiny fissures and pores of cementitious materials. Once these forces exceed the strength of the material, cracking occurs. During subsequent thawing, the water then moves through the cracks, expanding them further, to cause more damage when freezing occurs again.
Harper et al., U.S. Pat. No. 4,637,860, suggested that better freeze-thaw resistance could be achieved by autoclaving a cellulose fibercement mixture with silica sand additions. These inventors also recognized that silica sand additions reduced the density of formed sheet materials to a level below that necessary to achieve sufficient strength and freeze-thaw resistance. Accordingly, the '860 patent suggested compressing the wet mixture in a press to reduce its thickness and increase its density prior to autoclaving. Such a process has been proven to be effective in increasing the interlaminar bond strength (“ILB strength”) of fibercement boards when pressures approaching 30 bar are used. See Wierman et al., “The Effects of Pressure on Freeze-Thaw Durability of Fiber-Reinforced Cement Board” ( September, 2002).
While improvements to the processing of fibercement sheathing have been introduced, there have been some notable disadvantages associated with fibercement products compared to vinyl siding products. Specifically, even with cellulose fiber reinforcement, fibercement exterior panels are susceptible to cracking by nails and screws, especially along their edges. Moreover, while high pressure pressing has improved ILB strength, fibercement trim boards are still susceptible to freeze-thaw delamination failures, and could be improved in this area.
Accordingly, there is a current need in the cementitious exterior sheathing industry for a more durable cementitious siding and trim panel.